#include <stdio.h>

int main()
{
   char key[255],case_type,idx;
   
   // Yes, gets() is sloppy, but I don't care, this is just a neccessary step.
   // When actually using this, you can input the key any way you want.
   printf("Enter the string: ");
   gets(key);
   for (idx=0; idx < strlen(key); idx++) {
      // Only process alphabetic characters
      if (key[idx] < 'A' || (key[idx] > 'Z' && key[idx] < 'a') || key[idx] > 'z')
         continue;
      // Determine if the char is upper or lower case
      if (key[idx] >= 'a')
         case_type = 'a';
      else
         case_type = 'A';
      // Rotate the char's value, ensuring it doesn't accidentally "fall off" the end
      key[idx] = (key[idx] + 13) % (case_type + 26);
      if (key[idx] < 26)
         key[idx] += case_type;
   }
   
   printf("String run through ROT-13: ");
   puts(key);
   getchar();
   return 0;
}